1. Field
The disclosed concept pertains generally to power generation systems and, more particularly, to microgrids, such as, for example, distributed generation power systems. The disclosed concept further pertains to generator dispatching control methods for microgrids. The disclosed concept also pertains to load shedding control methods and systems.
2. Background Information
Fuel consumption and power reliability are two major concerns for power generation applications. In order to have a robust solution, the trend in power generation systems is developing toward distributed generation (DG) which includes conventional grid connect, conventional fossil fuel generation and renewable energy resources.
A typical approach for generator dispatching is to add or turn off a generator based on frequency droop characteristics of plural generators as a function of the grid load level in order to maintain system power reliability. As shown in FIG. 1, generator dispatching frequency thresholds are typically set within a normal load range. However, this approach does not always guarantee maximum efficiency and is not scalable for relatively larger intelligent distributed power generation systems. Moreover, if renewable energy resources are used, then the control strategy needs to be robust in order to provide desired flexibility.
In the generator dispatching control strategy of FIG. 1, which does not use an energy storage system, plot 2 is for both 30 kW and 60 kW generators (not shown) being on, while plot 4 is for only the 60 kW generator (not shown) being on. The plots 2,4 show per unit (pu) frequency (f) on the vertical axis and power output (kW and per unit power) on the horizontal axis. In the plot 2, when the power output decreases to 27 kW, or the frequency increases to 0.994 pu, the 30 kW generator is turned off at 6. Later, when the power output increases to 48 kW in plot 4, or the frequency decreases to 0.984 pu, the 30 kW generator is turned on at 8. The 60 kW generator keeps running as a master generator, turns on the 30 kW generator when the load exceeds a suitable threshold (e.g., without limitation, 80% of 60 kW), and turns off the 30 kW generator when load is below a suitable threshold (e.g., without limitation, 30% of 90 kW).
FIG. 2 illustrates demand dispatching through a simulation. The plotted signals represent the system frequency that is drooping based on a simulated load and based on the capacity slope of the operating generator. The dotted line is an indication of the operating state of a supporting generator. When the dotted line is “high” on the plot, a second supporting generator is online. When the dotted line is “low”, the second supporting generator is offline. At about time 3.0 a significant load step occurs. This load step draws greater power than the prescribed limit for the single operating generator. Additional generation is then applied causing the frequency to recover. At about time 7.0, the excess load is removed and the frequency begins to recover forcing the additional generation to turn off. As the transient from the load step continues, the frequency droops at 10 below the transition threshold and the additional generation is reapplied. After the frequency settles, following the load step transient, the additional generation is again commanded offline.
Hence, there is a need to prevent this generator cycling issue.
There is room for improvement in microgrids, such as, for example, distributed generation power systems.
There is also room for improvement in generator dispatching control methods for microgrids.
There is further room for improvement in load shedding control methods for microgrids.